The invention relates to a process for the preparation of fine-particle fillers, in particular those with flame-retardant action such as aluminium hydroxide or magnesium hydroxide, with improved free-flowability and improved bulk density behaviour, in particular after pneumatic conveyance through pipes, as well as polymer-coated fillers obtainable according to this process and their use as flame-retardant fillers in plastic or rubber compounds.
Fine-particle precipitated or also ground fillers often display a poor free-flowability. This impairs the flowability of the products, e.g. in compounding units or in those conveyance processes in which a good flow of the product is an essential criterion (e.g. silo discharge, flow in the internal mixer, etc.).
Although an improved free-flowability is achieved by adding dispersing agents customary in the trade, the products thus obtained generally have an electrical conductivity that is much too great for cable insulation applications.
A further disadvantage of these products is that after a conveyance process (e.g. through freefall or by means of air through a pipe) the bulk density sinks to a very low level. As a result, the compounding behaviour worsens at the same time in all current mixing units.
A further aspect is that the trend on the market is towards batch delivery. This means that even after a transport process, for example from the manufacturer""s silo to the silo vehicle and from there into the customer""s silo with subsequent conveyance into the service hopper, a high bulk density combined with good free-flowability is desirable.
Further important limiting conditions are that the properties of the plastic or rubber compounds prepared with these products must be unimpaired or only slightly impaired. These include in particular:
the rheological properties of the compounds
the mechanical properties of the compounds
the flame protection, where flame-retardant fillers are involved
the electrical properties of the compounds, in particular after ageing in water
silane-coupling to the filler, here primarily amino and vinyl silanes
the freedom from hard filler agglomerates
With regard to freedom from agglomerates, although agglomerates may be present in the filler, they must dissolve completely in the plastic during the compounding process.
In addition, the additive must not lead to a reduction in the throughput during filler production, for example during the spray-drying (e.g. because of a reduction in the solids content in the slurry or an increase in the viscosity of the slurry).
Furthermore an increased moisture affinity of the coated product is to be ruled out because this leads, e.g. in electrical insulation materials, to blister formation and thus among other things to a reduced dielectric strength.
The object of the invention was therefore to prepare fine-particle flame-retardant fillers which have a good free-flowability, display a high bulk density even after pneumatic conveyance, can be worked well into plastic compounds and the typical properties of which, in particular when used in cable compounds, are not disadvantageously influenced.
According to the invention, this object is achieved by a process for the preparation of surface modified fillers with improved bulk density stability, improved free-flowability and an average agglomerate size from 1 to 500 xcexcm. In this process an aqueous suspension of filler with a primary particle size in the d50-value of not more than 15 xcexcm with addition of 0.1 to 50 wt.-% (calculated as dry substance) of an aqueous polymer dispersion is spray-dried. The aqueous polymer dispersion consists of at least one thermoplastic polymer or copolymer and/or one rubber and optionally at least one self-crosslinking polymer or copolymer, dispersing agent and water.
It was found that by adding 0.1 to 50 wt.-%, preferably 0.3 to less than 1 wt. %, suitable additives to an aqueous filler suspension and then spray-drying, the desired properties can be achieved. The quantities given relate in each case to the dry substance, i.e. the filler on the one hand and the polymers or copolymers on the other. By suitable additives are meant according to the invention aqueous polymer or copolymer emulsions, at least one thermoplastic polymer or copolymer or a rubber (latex) or a mixture of several of the named polymeric substances being present.
It is surprising in particular that quantities of less than 1.0 wt.-% of the named polymers are already sufficient to achieve the effect according to the invention.
The polymer dispersions usually contain an emulsifier for the aqueous phase, anionic, cationic or also neutral emulsifiers being able to be used.
Natural or synthetic substances such as for example calcium carbonate, dolomite, barium sulphate, talc, china clay, mica, wollastonite, silicon dioxide (silica, silicic acid), aluminium oxide, magnesium oxide, aluminium hydroxide and magnesium hydroxide can be used as fillers.
Usable as thermoplastic polymers are for example styrene copolymers such as e.g. styrene/acrylonitrile copolymers (SAN) or SAN modifications such as e.g. acrylonitrile/polybutadiene/styrene graft polymers, or graft copolymers based on methyl methacrylate or polyacrylates such as e.g. acrylonitrile copolymers or graft copolymers of polymethyl methacrylate with polybutadiene. Copolymers of vinyl chloride and vinylidene chloride/acrylonitrile are also suitable.
Polyvinyl alcohol (PVOH) and also polyvinyl acetate (PVA) are preferred.
Also preferred are polymerized esters or copolymerized esters of acrylic acid with low alcohols, in particular those with C1-6 alcohols such as methyl acrylate, ethyl acrylate or butyl acrylate. These also include for example ethylene/ethyl acrylate copolymers (EEA), ethylene/methyl acrylate copolymers (EMA) and ethylene/butyl acrylate copolymers (EBA).
Polyethylene or copolymers of ethylene are also preferably used. Examples are: ethylene/vinyl chloride graft copolymers such as ethylene/vinyl chloride/vinyl acetate (EVCVA) and ethylene/acrylic acid copolymers (EAA).
Ethylene/vinyl acetate copolymers (EVA) and ethylene/vinyl alcohol copolymers (EVOH) are particularly preferred.
In a likewise preferred version, the polymer dispersion contains at least one self-cross-linking polymer or copolymer.
The polymer dispersions used according to the invention can be added for example in metered doses together with a dispersing agent required for the formation of the filler suspension. To prepare the filler suspension, e.g. the ground dry filler is liquefied with water and the dispersing agent, or in the case of synthetic (precipitated) fillers, the moist filler obtained after precipitation and filtration is used.
The spray-drying is advantageously carried out [with] a suspension the filler content of which is between 10 and 90 wt.-%, preferably between 40 and 65 wt.-%. Spray dryers customary in the trade can be used as can be obtained for example from Niro. Air is preferably used as desiccant gas, the quantity and entry temperature of which is advantageously such that an exit temperature of 100-150xc2x0 C. results.
The process according to the invention is particularly preferably used for the preparation of fillers based on aluminium and/or magnesium hydroxides. Both single hydroxides such as Al(OH)3, AlOOH and Mg(OH)2, and physical mixtures of these hydroxides or mixed hydroxides such as hydrotalcite or similar compounds can also be used.
A further particularly preferred filler is calcium carbonate.
A product with a primary particle size (given as d50 value) of not more than 15 xcexcm, preferably not more than 5 xcexcm, is advantageously used as filler. The free-flowable product, improved in respect of bulk density behaviour, i.e. conveyance-stable, which can be prepared from same according to the invention has an average agglomerate size of 1 to 500 xcexcm, preferably 20 to 200 xcexcm, and, according to the quantity of polymer dispersion used, a coating with 0.1 to 50 wt.-%, preferably 0.3 to less than 1 wt.-% of a polymer and/or copolymer.
The free-flowable fillers according to the invention are preferably used in a quantity of 5 to 90 wt.-% (relative to the overall weight of the compound) as flame-retardant fillers in plastic or rubber compounds, in particular in cable mixtures.
The following examples with aluminium hydroxide of different primary particle sizes illustrate the performance of the invention without being regarded as limiting it in any way.
The aluminium hydroxide used in the examples as starting materials had the properties summarized in the following Table 1:
Types OL-104/LE and OL-107/LE are products of Alusuisse Martinswerk Gmbh, Bergheim/Erft, Germany. SF4 ESD is an Alcan product. All types were used as product customary in the trade.
The mixing ratios of the compounds are given in the customary phr unit (1 phr=1 part by weight per 100 parts by weight polymer).